DESCRIPTION: A frequent and debilitating complication of breast cancer is the bone destruction that results when the cancer cells metastasize to bone. The underlying hypothesis for the proposed studies is that matrix-bound growth factors in bone may help define it as a fertile ground for the growth of breast cancer metastases and influence their bone-destructive potential. The goal of this proposal is to examine the role of bone matrix-bound transforming growth factor beta (TGFb) in regulating the bone destructive capacity of metastatic breast cancer cells through stimulating production of parathyroid hormone related protein (PTHrP), a powerful bone resorbing factor. Specific Aim 1 will determine whether bone matrix-bound TGFb is responsible for the increase in PTHrP production observed when breast cancer cells are grown on bone matrix. This will be done in vitro through the use of TGFb neutralizing antibodies and the use of bone matrix preparations from mice which lack the gene for TGFb-1. In vitro studies will be confirmed using an in vivo metastasis model to compare PTHrP production and osteolytic lesions by breast cancer cells in normal and TGFb-l deficient mice. In Specific Aims 2 and 3 the molecular mechanism for release of bone-matrix bound TGFb by breast cancer cells will be investigated. Since proteolytic cleavage of the latent transforming growth factor beta binding protein-1 (LTBP-1) is an important mechanism for release of matrix-bound TGFb by bone cells, Specific Aim 2 will determine whether a similar mechanism is used by breast cancer cells. This will be done by measuring release of LTBP-1 and TGFb by breast cancer cells grown on radiolabeled bone matrix using immunodetection methods. In Specific Aim 3 the proteolytic cleavage sites in LTBP-1 will be mapped. Antagonist peptides will then be designed and tested to identify peptides which will inhibit cleavage of LTBP-1 by breast cancer cells and therefore inhibit release of TGFb from bone matrix. Since latent TGFP must be activated in order to exert its effects on the breast cancer cells, Specific Aim 4 will examine the ability of the breast cancer cells to activate matrix-released latent TGFb. This will be done using bioassays for TGFb in conjunction with Western blotting. This study will provide a new approach for studying the complex interactions between breast cancer cells and the bone microenvironment and will address the general question of whether matrix-bound growth factors in host tissues influence the behavior of metastatic cancer cells. Blocking the release of bone matrix-bound TGFb by breast cancer cells may potentially be developed as a new treatment strategy to reduce cancer-associated osteolysis and alleviate the suffering of patients with bone metastatic breast cancer.